Hitherto proposed and designed commercial PFBC power plants are relatively small and have a power of up to about 200 MWe. Rectangular combustors have been used to obtain a geometrically simple construction of the tube system in the bed section of the combustor. A cleaning plane in the form of groups of cyclones for separation of dust from the combustion gases has been provided between the combustor and the surrounding pressure vessel.
The problem involved is that the hitherto proposed and designed PFBC power plants require a pressure vessel with a relatively large diameter in view of the geometrical shape of the combustor. In the first generation PFBC power plants, the advantage of the simple tube laying in rectangular combustors has made up for the additional cost of a larger pressure vessel. When doubling or multiplying the power it is important to increase the ratio between the cross-section areas of the combustor and the pressure vessel.